Spectroscopy devices are today widely used in commercial and scientific applications to identify and analyze substances of different kinds. In a typical spectroscopy device, a probe signal is sent to a sample and the reflected or transmitted part of the signal is then analyzed to capture the characteristic spectrum of the substance under test. The probe signal is in general electromagnetic radiation in the frequency range of infrared, visible light or microwave but also electrons or phonons can be used. The choice of the probe signal is related to the material properties that shall be investigated.
Recently, terahertz (THz) radiation (a range located between the microwaves and infrared light in the electromagnetic spectrum) has attracted the interest of the scientific and engineering community for its wide range of possible scientific and commercial applications. The fact that the vibrational modes of several molecules lie in this part of the spectrum and that water very easily blocks electromagnetic waves with those frequencies make THz radiation a suitable probe to investigate material properties which are usually not accessible with infrared probe signals or other types of probe signals.
Technical difficulties related to the possibility to detect and generate THz signals have for long time hindered the development of complete THz spectrometers, but nowadays several groups in the world have demonstrated the possibility to use THz radiation in such a way and even several commercial THz imaging and spectroscopic devices are available.
State-of-the-art THz spectroscopy devices are based on femto-second laser sources able to generate short light pulses to excite a Gallium Arsenide THz emitter. The generated THz radiation is sent toward a sample and the transmitted or reflected signal is then sampled using again the laser pulse. A schematic of transmission (FIG. 1) and reflection (FIG. 2) spectrometers operating according to this principle is shown in FIG. 1 and in FIG. 2, respectively. FIG. 1 shows a femtosecond laser 200, a scanning optical delay line 201, a terahertz transmitter 202, a plurality of parabolic mirrors 203, a sample 204, a terahertz detector 205, a current pre-amplifier 206, and an A/D converter and DSP (digital signal processing) unit 207. FIG. 2 shows a similar arrangement in reflection geometry. A device of this type is described in U.S. Pat. No. 6,747,736.
Other devices to carry out spectroscopy in such frequency band use backward wave oscillators (BWO).
Both solutions are based on discrete and bulky components. BWOs are considered very inefficient in the interesting frequency range and femto-second lasers remain both very bulky and expensive.
A different approach to the problem which is described in “All-electronic terahertz spectroscopy device with terahertz free-space pulses” (by J. S. Bostak et al. in J. Opt. Soc. Am. B, 11, No. 12, December 1994) uses non-linear transmission lines for the generation of very short pulses with spectral content reaching the THz range. A schematic of such an all-electronic spectroscopy device is shown in FIG. 3. As the devices described above, also this electronic THz spectrometer is based on discrete and thus rather bulky components. A signal at 6.0 GHz is generated by an external synthesizer 100 and amplified by a 30 dBm amplifier 103 before reaching a non-linear transmission line 106 integrated with an antenna. The source signal at 6.0 GHz is compressed by the non linear transmission line 106 to a THz pulse and transmitted by the antenna. The beam is collected and focused by a silicon lens and focused again by an external paraboloidal mirror 108. A similar arrangement is present at the receiver side (including another external paraboloidal mirror 108), where the detector is composed by an all-electronic two-diode sampler driven by the signal to be sampled and the signal from another external synthesizer 101 amplified by a 30 dBm amplifier 104 compressed by a non-linear transmission line 107. The IF signal (intermediate frequency signal) generated in this way is amplified (by low-noise amplifier 110) and visualized on an external instrument 109 (which can e.g. be formed by a spectrum analyzer or oscilloscope). A mixer 105 is provided outputting a trigger signal to the external instrument 109. A 10 MHz reference clock 102 for phase lock is provided by the external synthesizer 100 to the other external synthesizer 101. The functioning of this device is described in the cited reference. As for the approaches described above (the one based on lasers and the other based on backward-wave oscillators), this device is also based on several discrete components and makes use of two or three external measurement instruments. As a consequence, it is not suited for widespread, low-cost commercial applications.
From JP 2007-078 621, a sensing device for acquiring information on a specimen near an electric conductor section using surface plasmon resonance is known. Laser light is used for generating and coupling an electromagnetic wave including a THz frequency region and coupling the electromagnetic wave to the electric conductor section.
From WO 2008/105888, a sensing system is known that includes a radiation source and an integrated sensing probe disposed adjacent to the radiation source. The radiation source is for example a terahertz radiation source. The integrated sensing probe includes a substrate, a corrugated reflective surface and a coaxial wave-guide structure having a coaxial wave-guide tip. The corrugated reflector surface functions to enhance transmittance of the radiation through the tip by coupling the radiation to surface plasmon polaritons. A detection system, which can include collection optics, can be positioned below a sample in case of a partially transparent sample substrate, or beside the sample to measure scattered signals, or above the sample on the substrate side to measure reflected radiation, or it can be coupled to the resonator above the waveguide tip to measure the detuning of the resonance by the waveguide tip.
From WO2006/123153, a wave-guide structure for terahertz radiation is known, in which the surface-plasmon concept is applied for all-optical terahertz generation. A femto second pulsed laser beam is used for generating terahertz radiation. The terahertz radiation propagates along an interface of the wave-guide structure by means of a surface plasmon. An evanescent wave has a tail into air, which can be used for detecting or sensing a gas or a biomedical substance.